The present invention relates to an optical scanning device, which is used as an optical writing device of a digital image-forming device using an electrophotographic method such as a laser printer, a digital copier, a facsimile machine or other such method. The present invention further relates to an optical scanning device and an image forming apparatus including the optical scanning device which has an intensity distribution transforming optical component for transforming the intensity distribution of a light flux, and the image forming apparatus can be used in a digital outputting apparatus, for example, a digital copier, a printer, a facsimile machine or other apparatus.
A conventional optical scanning device includes a coupling lens, which couples a light flux emitted by a light source to form a parallel light flux, a weakly convergent light flux, or a weakly divergent light flux. An optical deflector deflects the light flux received from the coupling lens at a uniform angular velocity. A scanning image-formation optical system converges the light flux deflected by the optical deflector to form a beam spot on a surface to be scanned, i.e. a photosensitive body, and, thus, the surface to be scanned is scanned with the beam spot. Such an optical scanning device is used as an optical writing device in a digital image forming apparatus using the electrophotographic method such as a laser printer, a digital copier, a facsimile machine, or other apparatus.
In such an optical scanning device, in order to achieve high-density writing (more than 1200 dpi, for example), it is necessary to form a beam spot having a sufficiently small diameter.
In order to obtain a beam spot having such a small diameter, it is necessary to increase the NA of the optical system of the optical scanning device. Further, in order to obtain a stable small-diameter beam spot, it is necessary that the optical system provides a large depth of focus which tolerates possible component allowance (the curvature radiuses, thicknesses, refractive indexes) for deviations of optical components of the optical system, mounting errors, and environment variations (temperature, humidity).
Assuming that the intensity distribution on the exit pupil of the optical system is a Gaussian distribution, the allowable degree of depth of focus 2d is in proportion to the second power of the beam spot diameter w, as shown in the following expression;2dαw2/λ  (1)
In the above expression, λ represents the used wavelength. Thus, the allowable degree of depth of focus decreases sharply as the beam spot diameter is reduced. Therefore, when reduction of the beam spot diameter is attempted, the allowable degree of depth of focus decreases, and, as a result, it is not possible to obtain a stable small-diameter beam spot when the above mentioned component allowance deviations or environmental variations occur.
One solution to this problem is to generate the zero-order Bessel beam of the first kind and obtain a beam spot having a large allowable degree of depth of focus.
For example, Japanese Laid-Open Patent Application No. 9-304714 discloses an optical system providing a large allowable degree of depth of focus by arranging a shading member having a shading portion which shades a portion of a light flux on an optical path between a light source and an optical deflector.
Further, Japanese Laid-Open Patent Application No. 10-227992 discloses generation of a Bessel beam having an intensity distribution which is approximately in proportion to the second power of the zero-order Bessel function of the first kind, in a system in which a laser beam is incident on a diffraction optical component consisting of a binary optical component having an optical performance approximately equivalent to a conical prism.
However, in each of these systems, the intensity distribution of the beam is axially symmetric. Therefore, when the system is used as an optical system of an optical scanning device, it is difficult to independently set a beam spot in a main scanning direction (in which scanning is performed with a light flux) and in a sub-scanning direction (perpendicular to the main scanning direction).
In the above-mentioned system, a Bessel beam is obtained as a result of transforming the distribution of the amplitude term u1(y1, z1) of the following equation (2) into an arbitrary amplitude distribution. The intensity distribution thereof is expressed by the second power of the amplitude distribution. In the following equation (2), the direction of the optical axis is coincident with the x direction, the main scanning direction perpendicular to the optical axis is coincident with the y direction, and the subscanning direction also perpendicular to the optical axis is coincident with the z direction.                                           u            2                    ⁡                      (                                          y                2                            ,                              z                2                                      )                          =                                            j              ⁢                                                           ⁢                              ⅇ                                                      -                    ⅈ                                    ⁢                                                                           ⁢                                      k                    ⁡                                          (                                              x                        +                                                                                                            y                              2                              2                                                        +                                                          z                              2                              2                                                                                                            2                            ⁢                            x                                                                                              )                                                                                                          λ              ⁢                                                           ⁢              x                                ⁢                      ∫                          ∫                                                                    u                    1                                    ⁡                                      (                                                                  y                        1                                            ,                                              z                        1                                                              )                                                  ⁢                                  ⅇ                                                            -                      ⅈ                                        ⁢                                          k                                              2                        ⁢                        x                                                              ⁢                                          (                                                                                                    y                            1                                                    ⁢                                                      y                            2                                                                          +                                                                              z                            1                                                    ⁢                                                      z                            2                                                                                              )                                                                      ⁢                                  ⅆ                                      y                    1                                                  ⁢                                  ⅆ                                      z                    1                                                                                                          (        2        )            
The above equation (2) is expressed assuming that the intensity distribution u22(y2, z2) of the beam spot on the image surface is approximately in accordance with the Fraunhofer diffraction.
In the above equation (2):                u2(y2, z2): the amplitude distribution of the beam spot on the image surface;        u1(y1, z1): amplitude distribution on the pupil;        −ik(y1y2+z1z2)/2x: phase difference on the pupil (k represents the wave number); and        j/λ: Fresnel inclination coefficient (where λ represents the used wavelength).        
The expression of the Fraunhofer diffraction of the above equation (2) has the same meaning as that of Fourier transform expression, and the amplitude distribution u2(y2, z2) on the image surface is equal to that obtained from Fourier transform being performed on the amplitude distribution u1(y1, z1) on the pupil. Therefore, the expression of the Fraunhofer diffraction of the above equation (2) is referred to as a Fourier transformed image.
Further, in any method, when a Bessel beam is generated, side lobes develop. Therefore, when the sensitivity of the photosensitive body is high, image degradation such as resolution degradation and/or stain in background occurs.
FIGS. 1, 2A, and 2B show an example of an optical scanning device according to the related art. FIG. 1 shows an optical arrangement of the optical scanning device. In FIGS. 2A and 2B, the optical scanning device is shown in a condition in which the optical scanning device is expanded along an optical path of a light flux extending from a light source to a surface to be scanned. FIG. 2A shows the sectional view of the optical scanning device taken along a deflection plane (including the plane formed as a result of the light flux scanning the surface to be scanned), and FIG. 2B shows the sectional view of the optical scanning device taken along the plane including the optical path of the light flux and perpendicular to the deflection plane.
As shown in FIGS. 1, 2A and 2B, the optical scanning device 30 includes a light source 1 which emits a laser light, a first optical system 2 for directing the laser light emitted by the light source 1 to an optical deflecting portion 3, the optical deflecting portion 3 which deflects the light flux from the first optical system 2, and a second optical system 4 for forming a beam spot on the surface 5 to be scanned using the thus-deflected light flux. The above-mentioned first optical system 2 includes a collimating lens 21, an aperture 22, and a cylindrical lens 23. The second optical system 4 includes a spherical lens 41 and an fθ lens 42.
A process of optical scanning will now be described more specifically. The light flux emitted by the semiconductor laser 1, for example, is transformed into an approximately parallel light flux by the collimating lens 21, and passes through the aperture 22. It is also possible to use a coupling lens instead of the collimating lens 21, and to transform the light flux from the semiconductor laser 1 into a weakly divergent light flux or a weakly convergent light flux.
The light flux from the semiconductor laser 1 is transformed into the approximately parallel light flux, which is then converged into a line image elongated in the deflection direction by the cylindrical lens 23, and is directed to the deflection reflective surface of the polygon mirror 3. The light flux deflected by the polygon mirror 3 is incident on the scanning lens 41, and, the beam spot is formed on the surface 5 to be scanned. The characteristics of curvature of field and uniform-velocity characteristics of the scanning lens 41 are well corrected. Further, the light flux deflected by the polygon mirror 3 is first directed to a photodetection portion 6 by an optical-path changing mirror 8 via the scanning lens 41, and is used as a synchronization signal for detecting a position from which an image is written.
Generally speaking, the intensity distribution of the light flux emitted from a laser light source is a Gaussian distribution. At this time, the depth of focus Z of the light flux, which forms a beam spot having a diameter ω on a surface to be scanned, is expressed by the following expression;Z=kω2/λ  (a)
In the above equation (a), k represents a constant, and λ represents the wavelength.
As can be clearly seen from the above equation (a), as the diameter ω of the beam spot is reduced, the depth of focus Z decreases at the rate of the second power of the diameter ω of the beam spot.
The diameter ω of the beam spot is expressed by the following equation:ω=K λ/NA  (b)
In the above equation (b), K represents a constant, λ represents the wavelength, and NA represents the numerical aperture.
Recently, as the resolution of an image outputting apparatus which uses a laser as a light source such as a laser printer is increased and the quality of images obtained there from is increased, it is necessary to reduce the diameter of a beam spot on a surface to be scanned, that is, the surface of a photosensitive body in an example of a laser printer.
However, as shown in the equation (a), the depth of focus Z is determined to be in proportion to the second power of the diameter ω of a beam spot in the case of a Gaussian beam, reduction in the diameter ω of the beam spot results in decreases in the depth of focus Z, and, thereby, it is difficult to satisfy the allowable range for practical use.
In order to solve the problem, Japanese Laid-Open Patent Application No. 5-307151 discloses an optical scanning device in which a beam spot on a photosensitive body is formed by a Bessel beam.
The Bessel beam is a non-diffracting beam by using which it is possible to reduce the diameter of a beam spot and to increase the depth of focus in comparison to the above-described Gaussian beam. The Bessel beam has the intensity distribution approximately in proportion to the second power of the zero-order Bessel function of the first kind. With regard to the Bessel beam, see “Exact Solutions For Non-diffracting Beams’, written by J. Durnin, Vol. 4, No. 4/Apr. 1987/J. Opt. Soc. Am. A, page 651.
Methods of generating the Bessel beam include using a ring-shaped thin slit (see ‘Diffraction-Free Beams’, written by J. Dumin et al., Physical Review Letters, Vol. 58, No. 15, 13 Apr. 1987, page 1499), and using an axicon prism (see ‘Long-Range LaserBeam Spot Formation By An Axicon Prism’, written by Satoshi Kawata et al., Proceedings of Spring Lecture of Applied Physics Society (1990), page 829) and others.
The above-mentioned Japanese Laid-Open Patent Application No. 5-307151 discloses an optical scanning device, which performs image formation on a surface of a photosensitive body using a Bessel beam having an intensity distribution approximately in proportion to the second power of the zero-order Bessel function of the first kind, obtained from a laser light.
However, because the Bessel beam develops large side lobes, the image quality is degraded.
In order to reduce the side lobes, Japanese Laid-Open Patent No. 6-148545 discloses an optical scanning device, which generates an eccentric Bessel beam, and cuts off the side lobes using a slit member. In this optical scanning device, the slit member having a slit in a direction, which coincides, with the deflection direction is arranged in proximity to a surface to be scanned.
However, it is difficult to adjust the position of this slit member properly. Further, by using the slit, the quantity of light is greatly reduced. There is another method in which an axicon prism is used for reducing the side lobes. However, in this method using the axicon prism, it is necessary to mount the axicon prism with high accuracy, and thereby, it is difficult to achieve mass production of optical scanning devices using the axicon prisms.
Japanese Laid-Open Patent Application No. 9-243945 discloses an optical scanning device in which stop means and shading means are provided between a collimating lens and a cylindrical lens. By using the simple means, the diameter of a beam spot is reduced, and the depth of focus is enlarged.
However, also in this method, a portion in the vicinity of the center of a light flux is cut off, and, thereby, the quantity of light is reduced.
Further, recently, consideration of environmental factors is required, and, recycling is being performed for OA equipment such as a copier. Accordingly, designing of structures and components which are suitable for recycling is becoming advanced, and, also, designing of components, which can be used in common for various devices, is being further developed.